Conventional lubricants include petroleum-based esters and are known to contaminate soil and water through fluid losses in lubrication systems. They are widely used in the automotive industry and in a variety of other industrial applications. In 2002, the total lubricants market for Western Europe was 5,020,000 tons per year; and for the United States it was 8,250,000 tons per year. See L. R. Rudnick and S. Z. Erhan, “Natural Oil as Lubricants,” in Synthetics, Mineral Oils, and Bio-Based Lubricants: Chemistry and Technology, CRC-Taylor and Francis, New York, Chapter 21 (pp. 353-360), 2006.
In recent times, a pursuit for “greener” technology and carbon-neutral products has led to an increasing demand for biolubricants such as bioesters derived from vegetable oils and/or animal fats, the annual growth rate of these biolubricants being over 10%. The biolubricants market in 2000 was 50,000 tons per year in Western Europe and 25,000 tons per year in the United States (Whitby, “Market Share of bio-lubricants in Europe and the USA,” Lipid Technology, vol. 16, pp. 333-337, 2000). If the quality of biolubricants is improved, this improvement will accelerate their demand beyond the traditional “total loss lubricant” sector. Simultaneously, with dramatic fluctuations in crude oil prices, there has been an increased focus on alternative sources of energy. Annual production of biodiesel (primarily fatty acid methyl ester (FAME)-based) in the United States for 2001 has been estimated at 57-76 million liters, with European production more than 10 times that amount. It is predicted that, in the United States alone, production will reach 1.3 billion liters annually by 2011 (Haas, “Improving the economics of biodiesel production through the use of low value lipids as feedstocks: vegetable oil soapstock,” Fuel Processing Technology, vol. 86, pp. 1087-1096, 2005). Thus, there is a steadily increasing level of interest in the growing market of biolubricants and biodiesel, and this interest is likely to be sustained for the foreseeable future.
1. Biomass Precursors
Typically, precursor material (i.e., feedstock) for both biolubricants and biodiesel (a representative biofuel) is a triglyceride-bearing material such as vegetable oil and/or animal fat (tallow). A key concern with the use of these feedstocks is their generally poor oxidation stability. In the case of biolubricants, oxidation causes polymerization and degradation. Polymerization increases the molecular weight of bioesters, which in turn leads to increased viscosity, gelling, and a general loss of lubricant functionality. Degradation leads to degradation (breakdown) products that are volatile, corrosive, and which can diminish the structure and properties of the lubricants. See, e.g., Wagner et al., “Lubricant base fluids based on renewable raw materials: Their catalytic manufacture and modification,” Applied Catalysis A, vol. 221, pp. 429-442, 2001.
Biodiesel (a primary biofuel) prepared from vegetable oil can deteriorate due to oxidative polymerization, which can lead to formation of insoluble products that can cause problems within automotive fuel systems—especially injection pumps. The ease of oxidation generally depends on the fatty acid composition of the vegetable oil (see Falk et al., “The effect of fatty acid composition on biodiesel oxidative stability,” vol. 106, pp. 837-843, 2004). Unsaturated fatty acyl chains react with molecular oxygen to form free radicals that lead to polymerization and fragmentation. The rate of oxidation depends on the degree of unsaturation of a fatty acyl chain. If the rate of oxidation is normalized to 1 for a saturated fatty acid such as stearic acid, it is nearly 10 for oleic acid (single double bond), 100 for linoleic acid (2 double bonds), and 200 for linolenic acid (3 double bonds). This instability is attributed to the presence of allylic methylene groups between the double bonds. In addition, under thermal conditions the double bonds in polyunsaturated fatty acids isomerize to form conjugated acids, which are more susceptible to polymerization. See, e.g., Kodali, “High performance ester lubricants from natural oils,” Industrial Lubrication & Tribology, vol. 54(4), pp. 165-170, 2002.
Another concern with the use of vegetable oils as precursor material for biolubricants and biofuels is the potential for poor low temperature flow behavior for the resulting biolubricants and biodiesel derived therefrom (see Wagner, vide supra). Saturated fatty acids generally have a high pour point, implying that they (and any esters derived therefrom) may not be suitable for applications at low temperatures because they can freeze and/or otherwise no longer flow sufficiently. Unsaturated fatty acids have lower pour points because they have a disorganized crystal lattice due to the presence of one or more double bonds. To illustrate this point further, stearic acid (a fully saturated fatty acid) freezes at 70° C., oleic acid (a monounsaturated fatty acid with a single double bond in its fatty chain) freezes at 14° C., and linoleic acid (a polyunsaturated fatty acid with two double bonds in its fatty chain) freezes at −5° C.
The two aforementioned concerns (oxidation stability and low-temperature flow properties) are largely in conflict, as they lead to situations where better oxidation resistance properties for these applications (biolubricants and biodiesel) requires the use of vegetable oil rich in saturated fatty acid, but better flow behavior mandates the use of unsaturated fatty acids. To address any such conflict, a balance must often be found whereby the biolubricant or biodiesel composition is optimized for a particular application in terms of the compositional ratio of unsaturated to saturated molecules. For a pictorial representation of these conflicting optimization parameters, see FIG. 1.
Fatty acid composition and distribution vary widely among various vegetable oils (after hydrolysis). Referring to Table 1 (FIG. 2), at one end of the spectrum is a vegetable oil with high percentage of saturated fatty acids (palm oil), and at the other end is a vegetable oil with high percentage of polyunsaturated fatty acids (rapeseed oil). Ideally, a feedstock rich in oleic acid will lead to better quality esters and therefore better quality biolubricants or biodiesel. As feedstock selection is often based on its price and availability, oleic acid enrichment can have beneficial results and produce high oleic content bioesters regardless of the initial character of the triglyceride-bearing feedstock. Such enrichment permits a tailoring of properties to yield a well-defined biolubricant/biofuel from any of a number of different feedstocks of differing character.
2. Generation of Free Lipids
Existing strategies for such above-mentioned separations first require conversion of triglycerides into free lipids. Depending on the approach taken, the resulting free lipids are in the form of either fatty acids or fatty esters. Referring to FIG. 3, Approach 1 illustrates the transesterification of triglycerides with methanol to yield multiple methyl ester species and glycerol (glycerin). Still referring to FIG. 3, where the multiple methyl ester species comprise methyl oleate (1), methyl linoleate (2), and methyl stearate (0), such species having a number (n) of carbon-carbon double bonds (—C═C—) in their fatty (aliphatic) chains as shown in parentheses. Such methyl ester species can be separated from glycerol and treated to yield products that are substantially enriched in each of the individual fatty esters.
Referring again to FIG. 3, as an alternative to the above-described transesterification approach to generating free lipids, Approach 2 illustrates the hydrolysis of triglycerides to yield glycerol and free fatty acids. Similar to the separation of the fatty esters by degree of saturation, the fatty acids can also be analogously separated and subsequently esterified.
3. Separation of Fatty Acids/Esters
Some important procedural factors/elements, from the standpoint of preparation of good quality biolubricants, include, but are not limited to, an approach to separate oleic acid (monounsaturated) from linoleic acid (polyunsaturated), an approach to separate stearic acid and other saturated fatty acids from the unsaturated ones, an approach to separate methyl oleate (monounsaturated) from methyl linoleate (polyunsaturated), and an approach to separate methyl stearate and other esters of saturated fatty acids from unsaturated esters.
Often, the techniques to separate fatty acids are also applicable for the corresponding esters. This implies that both the approaches discussed above can be considered for the development of biolubricants. Secondly, the separation of saturated molecules from unsaturated molecules is relatively easy due to significant differences in their freezing point. The most challenging step is to separate linoleic acid from oleic acid (or the corresponding esters). In the following section, past work is reported on fatty acid separations, with emphasis on separating linoleic (polyunsaturated) acid from oleic (monounsaturated) acid.
4. Current Separation Technologies
Distillation, as a technique, has been reported for the separation of fatty acid methyl esters derived from vegetable oil. Both fractional distillation and molecular distillation have been applied for fatty acid separation. Weitkamp reported separating out methyl esters of cottonseed oil-derived fatty esters through an application of amplified distillation carried out at 2 mm Hg (Torr) pressure. The cuts were obtained at near 120° C. and 160° C. This technique could separate saturated esters from unsaturated esters, but no separation of unsaturated fatty acid esters, by degree of unsaturation, was obtained. See Weitkamp, “The Amplified Distillation of Methyl Esters of Fatty Acids,” J. Am. Oil Chem. Soc., vol. 24, pp. 236-238, 1947. A lot of work has been reported in the literature on this technique for analysis of fats and oils—particularly in the first half of the twentieth century. Molecular distillation is another technique aimed at reducing the tortuous path between the boiler and the condenser in a conventional distilling apparatus. It is carried out at very low pressures (0.01 to 0.001 mm Hg). Lambou and Dollear were able to prepare high purity linoleic acid by molecular distillation (Lambou et al., “Modified Thiocyanogen Reagent and Method,” Oil & Soap, vol. 22, pp. 226-232, 1945). The process of separation through distillation, however, is an energy intensive process.
Low-temperature crystallization is a widely applied process that was developed as a more efficient alternative to the moderately efficient distillation process developed in the 1930s for the separation of mixed acids and esters derived from natural fats. A look at the properties of fatty acids and their ester analogues indicates that the melting points of stearic/oleic/linoleic are widely separated and can therefore be considered for low-temperature crystallization. Bertran described a method to separate an oleic acid and linoleic acid mixture by crystallizing three times from acetone solution (1:1) at −10° C. to −15° C. and separation of the crystalline solid acid at −20° C. (Bertran, “The preparation of pure oleic acid,” Rucueil des Travaux Chimiques des Pays-Bas et de la Belgique, vol. 46, pp. 397-401, 1927). The product was a highly pure oleic acid. Low temperature crystallization has been tried successfully to separate saturated acids from unsaturated ones. Hartsuch made a comparison of the lead salt-alcohol, barium salt-alcohol-benzene, and low temperature crystallization methods for the separation of oleic acid from a saturated and unsaturated acid mixture and concluded that the efficiency of the low temperature crystallization process was the highest (Hartsuch, “A Study of the Methods of Separation of Oleic Acid from Saturated Acids and Linoleic Acid with Observations on the Preparation of Oleic Acid,” J. Am. Chem. Soc., vol. 61(5), pp. 1142-1144, 1939). Wheeler and Riemenschneider have used low temperature crystallization for separation of fatty acid methyl esters as well (see Riemenschneider et al., “Methods of Analysis of Mixtures of Oleic, Linoleic and Saturated Esters and Their Application to Highly Purified Methyl Oleate and Methyl Linoleate,” Oil & Soap, vol. 16, pp. 219-221, 1939).
Adsorption techniques can find significant application in the separation of unsaturated polyunsaturated fatty acids (as well as their methyl esters) based on their degree of unsaturation. UOP has patented a process for separating fatty acid esters by selective adsorption using an X or Y zeolite adsorbent with an exchangeable cationic site with metal ions from Group 1A (U.S. Pat. No. 4,049,688). This process uses high temperatures and pressures (e.g., 125° C. and 50 psig). Another UOP process for separating oleic acid from linoleic acid using a molecular sieve comprising silicalite is described in U.S. Pat. No. 4,529,551.
Salt-solubility methods are largely based on the proclivity of saturated and unsaturated fatty acids to form salts with metallic ions, the solubility of such salts in water and organic solvents varying with the nature of the metallic ion and the chain length, the degree of unsaturation, and other characteristics of the fatty acid component. The method is not easily quantifiable, and it is primarily utilized to generally remove saturated fatty acids from unsaturated ones. The most common method based on salt solubility is the lead salt-alcohol method which is based on the differential solubility of lead salts or soaps of fatty acids in diethyl ether or ethanol. The process is very non-specific, and it is primarily applicable for removing saturated fatty acids (for which alternative options are available). Moreover, for environmental and toxicological reasons, the use of lead salts is highly undesirable—especially when the end product is a biolubricant or a biodiesel.
Phase separation is another technique employed to concentrate/enrich the content of a particular type of fatty acid in one phase. Partition coefficients of fatty acids in different solvent systems have been reported in the literature (see, e.g., Mehta et al., “Preparation and Properties of Activated Urea,” Grasas Aceites (Sevelle, Spain), vol. 10, pp. 27-29, 1959). For example, in a mixture of heptane (4 volumes) and acetonitrile-methanol-acetic acid (1 volume each), oleic acid partitions 1.9 times in the heptane phase while linoleic acid partitions only 0.9 times. This technique can therefore be used to concentrate oleic acid in n-heptane. The key concern is the use of some solvents that are considered particularly useful for this process (e.g., acetonitrile). Similarly, methyl esters of oleic acid and linoleic acid can be separated using a pentane-hexane/acetonitrile. Some solvents such as acetonitrile and formamide are highly toxic. An 80% ethanol solution can be considered as a substitute for acetonitrile.
Partial hydrogenation of fatty acids is another approach to enriching vegetable or vegetable-derived oil in monounsaturated oleic acid. The process typically requires passing hydrogen gas under 30-40 psi pressure and a noble metal-based catalyst through an oil containing a mixture of fatty acids and partially hydrogenating the polyunsaturated fatty acid molecules to yield oil rich in monounsaturated fatty acid. Catalytic hydrogenation to produce mono-unsaturated fatty acids has been reported by Behr et al. in U.S. Pat. No. 5,354,877, where poly-unsaturated fatty acids were hydrogenated at 0-150° C. and 800-1500 hectopascals (hPa) in the presence of a catalyst. The oleic content reported in the product was over 90%.
Complexation with urea requires dissolving a known amount of oil-derived (e.g., from hydrolysis) fatty acids in a boiling solution with a proportionate amount of urea in methanol. Crystals of urea complexes form as soon as the container is removed from the steam bath used to heat it. The mixture is cooled to 0° C. overnight and subsequently filtered to yield a urea complex. The urea complex is boiled in a large volume of water to dissolve urea and yield an oleic acid-enriched urea mixture. The process can be used for methyl esters as well. D. Swern and W. E. Parker were able to enrich a fatty acid mixture with oleic acid from 45% to 78% using this method. See U.S. Pat. No. 2,838,480.
Adsorption by π(pi)-complexation has been reported as a promising alternative to cryogenic distillation to separate olefins from paraffins. Ag+ and Cu+ ions dispersed on resins, zeolites, silica, and pillared interlayer clays have been used to carry out these separations. A separation of olefins from paraffins using ionic liquids containing silver salts has also been reported (U.S. Pat. No. 6,623,659). Selective adsorption has been used in chromatography columns to separate fatty acid methyl esters of oleic acid from linoleic acid. See Dobson et al., “Silver Ion Chromatography of lipids and Fatty Acids,” J. Chromatography B, vol. 671, pp. 197-222, 1995; Emken et al., “Separation of Saturated, Unsaturated, and Acetylenic Fatty Acid Isomers by Silver Resin Chromatography,” J. Am. Oil. Chem. Soc., vol. 55, pp. 561-563, 1978.
Some of the advantages and disadvantages of the above-described techniques have been summarized in Table 2 (FIG. 4). Particularly in view of the limitations of these above-described techniques, a more facile and/or more tailorable method of fatty acid/ester separation by degree of unsaturation would be extremely useful—particularly to the extent that it could be integrated with existing processes and subprocesses for the production of biofuels and biolubricants.